1. Field of the Invention
The invention relates generally to vertical diode lasers, and particularly to vertical diode lasers having at least one patterned layer, and also to the production and use thereof.
2. Description of the Related Art
From as early as the mid nineteen eighties diode lasers have assumed a salient position in laser technology. High efficiency, compactness and very simple handling are the most important advantages of the diode laser. They have been used hitherto primarily in communication and information technology, in fiber-optic networks and CD players through to laser printers and professional printing systems.
The vertical diode laser (vertical cavity surface emitting laser; VCSEL) has proved to be particularly advantageous, this laser being distinguished primarily by simpler mounting and cooling and also better optical properties of the laser light in comparison with so-called edge emitters, which emit the laser light parallel to the substrate, that is to say laterally. Further advantages are a lower energy consumption and a more compact electrical circuitry, which affords price advantages particularly in the case of mass production applications, and also the possibility of use for on-wafer testing.
Unlike conventional edge emitters, however, conventional vertical diode lasers do not have a defined direction of polarization. Whereas in edge emitters light is propagated parallel to the quantum film and the transition matrix elements thus differ for the two directions of polarization, in VCSELs the propagation wave vector is perpendicular to the quantum films, as a result of which the gain is identical for all conceivable polarizations. No polarization is likewise preferred by the cylindrical resonator of conventional VCSELs.
The linear electro-optical effect causes, in VCSELs, a deformation of the refractive index ellipsoid along the crystal axes [011] and [0-11] with the consequence that the polarization of the fundamental mode of a vertical laser diode is usually oriented along one of these two crystal axes. The next higher mode is generally polarized orthogonally with respect to the fundamental mode. A current change or an additional external strain may result in an abrupt change in the polarization of a mode between the two crystal axes. These so-called polarization jumps are associated with a shift in the emission wavelength on account of the electro-optical effect. These polarization jumps generally limit the use of VCSELs in polarization-dependent optical systems. By way of example, the polarization jumps are disadvantageous in printing systems in which media having a different absorption for different polarizations are printed. Moreover, the polarization jumps lead to an increased noise in optical data transmission. In addition, the polarization jumps may prevent use in spectroscopy due to the influence on the emission wavelength.
U.S. Pat. No. 5,995,531 discloses, for example, forming the upper mirror of a VCSEL with a cross section that deviates from the circular form, for example elliptically, in order thus to stabilize the polarization. What is disadvantageous about this is that the coupling into an optical fiber, for example during optical data transmission, is made more difficult in the case where a cross section of the upper mirror of a VSCEL deviates from the circular form.
In order to stabilize the polarization, it is furthermore known to apply a vertical laser diode on more highly indexed substrates, for example on a [311] substrate. However, this leads to an impairment of the other laser properties and permits only one predetermined direction of polarization.
In addition to many other unsatisfactory approaches that are not described here, the use of surface gratings for polarization control has already been attempted earlier, but this, too, has not led to satisfactory results heretofore.